1. Field of the Invention
This invention relates to fluoropolymer compositions.
2. Introduction to the Invention
Fluoropolymers are well known and are useful in a broad range of applications. For example, many fluoropolymers can be melt extruded around conductors to provide electrical insulation thereon. Ethylene/tetrafluoroethylene copolymers (usually referred to as ETFE or ETFE polymers), and copolymers of tetrafluoroethylene with hexafluoropropylene or with a perfluorinated vinyl ether, are particularly useful for this purpose; ethylene/chlorotrifluoroethylene copolymers (usually referred to as ECTFE or ECTFE polymers) have also been used. For low loss insulation, the fully fluorinated copolymers are preferred, since the electrical insulation properties of ECTFE are poor by comparison. Tetrafluoroethylene polymers (usually referred to as PTFE) have excellent electrical insulation properties, but generally have very high molecular weights and crystallinities, such that the polymers cannot be melt extruded. However, it has been proposed to blend PTFE (especially PTFE which has been irradiated to reduce its molecular weight) with other fluoropolymers having a relatively low crystallinity, thus preparing a composition which can be melt extruded but which has improved properties resulting from the presence of the PTFE. U.S. Pat. No. 4,624,990, the disclosure of which is incorporated herein by reference for all purposes, discloses melt-shapeable compositions which comprise (a) a first melt-shapeable fluoropolymer having a crystallinity less than 45%, preferably less than 30%, and (2) a second fluoropolymer which has a crystallinity of at least 50% and which cannot be melt extruded on its own, preferably irradiated PTFE. The first fluoropolymer is preferably a copolymer of tetrafluoroethylene and one or more perfluorinated comonomers, e.g. hexafluoropropylene or perfluorovinyl ether, but ECTFE polymers are also referred to. The ratio of first fluoropolymer to second fluoropolymer is preferably 0.3:1 to 2.5:1, particularly 0.3:1 to 1:1. The patent refers to the possibility of foaming the composition as it is shaped, e.g. to provide low loss insulation.